Aroma, flavour and taste are considered to be the three important parameters for determining the quality of spices or condiments. The aroma and flavour of a spice depends upon the presence of volatile components, which are perceived through the olfactory epithelium present in the nose. The olfactory substances are primarily mono-, sesqui- or diterpenes, low molecular weight hydrocarbons, heteroacyclic or heterocyclic compounds. The taste is perceived by the taste buds present on the surface of the tongue. The interactions between the pungent molecules and receptors present on the surface of the tongue are the main cause of generation of hot sensation or thermogenecity which is associated with pungency. More pungent the substance, more is the duration of the feeling of hotness. This feeling of hotness may also be perceived on other tender parts of the skin. Most of the known natural pungent substances are aromatic in nature. Some of the most commonly known natural pungent substances include piperamides such as piperine from black pepper (Piper nigrum), capsaicinoids such as capsaicin and dihydro-capsaicin from red chillies (Capsicum annum) and gingeroids like gingerol from ginger (Zingiber officianalis) etc. The chemistry and properties of pungent compounds from natural sources has recently been reviewed (Nakatani, Nobugi; Koryo, 1995, 185, p 59–64, Chem. Abs. 123, 5520 n).
These pungent principles which possess anti-oxidant properties are also associated with many physiological actions viz piperamides, capsaicinoids as well as gingeroids are reported to possess anti-inflammatory properties (Lewis, D. A. in Anti-inflammatory drugs from plant and marine sources; Birkhauser Verlag, Berlin, 1989, p 216–220; Janusz, J. M., J. Med. Chem. 1993, 36, p 2595). Both piperine and dihydrocapsaicin were found to interact irreversibly with hepatic drug metabolizing enzymes thereby inhibiting their activity as indicated by prolongation of pentobarbital sleeping time in rats (Sush Young-Joon, A, et al., Life sciences, 1995, 56(16), p 305–31 1; Atal, C. K. et al., J. Pharmacol. Exp. Therp. 1985, 232, p 258–262).
Only few reports have appeared in the literature related to the evaluation of structure with respect to pungency. Wilbur Scoville developed direct subjective method of analysis of capsaicin and related extracts for their pungency evaluation in 1912. In this method potency of pepper was measured as heat units by diluting the extracts until pungency was just detected after placing a drop on the tongue. Pure capsaicin was thus assigned 107 units and jalapeno 103 units (Scoville, W. J., Am. Pharm. Assoc. 1912, 1, p 453–454). Indirect evaluations of pungency through the measurement of physiological effects have also been employed. For example Watnabe et al. reported the assessment of pungency related thermogenecity to structure of capsaicin analogues by correlating it with adrenal catecholamine secretion in rats (Watnabe, Tatsno et al. Life. Sci., 1994, 54(5), p 369–374; Chem. Abs., 120: 94792f). Astringent and pungent substances were studied using a multichannel taste sensor by observing the changes in electric potential in lipid membranes. However pungent substances were found to have no effect on the membrane potentials (Iiyama, Satoru et al., Chem. Senses, 1994, 19(1), p 87–96; Chem. Abs. 120: 295156).
Known natural or synthetic pungent substances are invariably oxygenated aromatic compounds with an alkyl side chain and a heteroatom comprising preferably an amide linkage in the side chain. Some of these compounds were found to possess hot pungent characteristics on preliminary evaluation. On the basis of above observation several new series of aryl alkenoic acid amides were synthesised and their structure and pungency relationship has also been established. Another important aspect of the development of these compounds is the possibility of their utilisation as new test models for the study and management of the phenomenon of pain and inflammation. These observations are based on the recent work carried out by the David Julius et al. who have identified the capsaicin receptor by isolating the complimentary DNA encoding the binding protein (Julius, D. et al. Nature, 1997, 389, p 816–824). Isolation and identification of the receptor protein is an important development for the study of new pharmacological targets which will help in understanding the phenomenon of chronic pain caused by arthritis, spinal cord injury or diabetic neuropathy. It is expected that vanilloid receptor and the receptor for the synthesised compounds of general formula 1 may turn out to be the same. Therefore, the development of these novel compounds may provide a diverse and effective alternate to capsaicin and related vanilloids.